In electrowinning nowadays, particularly in zinc electrowinning, cathode plates made of aluminium are used, which are connected to support bars. The cathode is lowered into the electrolysis cell by the support bars so that one end of the support bars is located on top of the busbar at the edge of the cell and the other end on top of the insulation. To ensure good electrical conductivity, a contact piece made of copper is attached to the end of the aluminium support bar, and the contact piece is set on top of the busbar. The lower edge of the contact piece is either straight or a notch is made there and the support bar is lowered on top of the busbar at the notch. Both side edges of the notch form a contact point, creating a double contact between the support bar and the busbar. When the lower edge of the contact piece is straight, an plane-type contact is formed between the busbar and contact piece. A straight contact piece is used particularly in large cathodes, known as jumbo cathodes.
The copper contact piece can be attached to the aluminium support bar for example by various welding methods. One of these methods is described for instance in U.S. Pat. No. 4,035,280. The patent also mentions that copper contact pieces may be coated with silver before welding. The publication does not, however, give any further description of how the coating of the contact pieces is carried out apart from this one sentence.
The Japanese application 55-89494 describes another method of manufacturing an electrode support bar. The actual support bar is aluminium and to its end is welded a contact piece with an aluminium core and a copper shell. The contact pieces are given their polygonal form by high-pressure extrusion.
In the prior art, the above-mentioned U.S. Pat. No. 4,035,280 proposed that the contact pieces be coated with silver. Clearly, silver improves the electrical conductivity of the copper piece, but coating of the whole contact piece does not suit its purpose and is costly. The combined aluminium and copper extrusion mentioned in the Japanese application does not necessarily achieve a metallurgical joint between the copper and aluminium, so the joint is electrotechnically weak and is damaged as the electrolyte penetrates the interface.
In zinc electrowinning, the rapid wearing of contact pieces in aluminium cathode support bars and in particular their contact surfaces poses a problem. The cause may mainly be the oxidation of copper into its oxide and the corrosion of oxide into copper sulphate under the effect of the electrolyte. Copper sulphate formed on the contact surface further weakens the electrical conductivity of the contact piece.
The method according to the invention relates to the obtaining of a good contact surface on the aluminium cathode bar used in electrolysis, particularly in zinc electrowinning, onto the end of which bar a separate contact piece is attached. The material used for the contact pieces is copper. According to the method now developed, the area on the lower surface of the support bar contact piece, the contact surface, which is to touch the electrolysis cell busbar, is coated with a highly electroconductive metal or metal alloy such as silver or silver alloy. The cathode is formed of a cathode plate and support bar, wherein the cathode plate is immersed in the electrolysis cell and the support bar is supported by its ends on the sides of the electrolysis cell so that the contact piece is located on top of the busbar. When a metallic joint is formed between the support bar contact piece and the coating made on its lower surface, the problems caused by wear or oxidation of the lower surface of the contact piece are avoided. The invention also relates to the cathode support bar used in electrowinning manufactured with this method, where the contact piece at the end of said bar forms a highly electro-conductive layer, in particular in the place, the contact surface, where it comes into contact with the electrolysis cell busbar.
The essential features of the invention appear in the appended claims.